1. Field of the Invention
The present invention relates generally to a dryer system, and a method of drying, that has improved throughput capacity over conventional dryer systems, and more particularly to a dryer system combining a high angle agglomerate catcher with optional overflow, a cylindrical dewatering feed chute for increased dewatering capacity, and a centrifugal dryer with a modified rotor design with positionally and structurally modified lifters and an efficient circumferential foraminous membrane. The present invention increases pellet input and output rates while maintaining the ability to achieve desired pellet moisture content.
2. Description of the Prior Art
The generally independent processes and equipment in the extrusion, pelletization, and drying processes of polymeric material are known. Over time, the demand for dryer systems with high drying capacities has increased.
Drying systems typically include both an agglomerate catcher and dewaterer that receive the slurry of water and plastic particulates in advance of the centrifugal dryer. The agglomerate catcher catches, separates and subsequently discharges agglomerated particulates before the slurry enters the dewaterer. The dewaterer then separates the bulk water from the particulates prior to entrance of the wet particulates to the dryer. Once the bulk of the water has been removed from the particulates, the particulates still include surface moisture that is removed by the centrifugal dryer during elevational and centrifugal movement of the particulates by rotation of the rotor within the dryer and circulation of air by a blower.
An agglomerate catcher is disclosed in U.S. Pat. No. 2,133,974, wherein blades are attached only at the uppermost or inlet end of a casing at a small angle relative to the direction of the inflowing liquid. The blades are arranged lengthwise with the direction of flow and are curved upwardly into the flow in their distal portions. The angle between the edges of the blades and the axis of flow at the distal end is considerably greater than that at the proximal or inlet end. The lower ends of the blades are not secured, and the distance between the blades depends somewhat on the nature of the material and the size and nature of the solid objects. The length of the blades is disclosed as being sufficient to extend completely through the flow. A rounded and smooth baffle can be placed between these blades and the angularly inclined screen that is designed to receive and drain the residue that may slide down. The screen can be pivotally mounted on the casing to allow variation in the angle of inclination as needed.
U.S. Pat. No. 4,447,325 discloses a dewaterer including a vertical dewatering section attached to an angled feed chute screen. The vertical dewatering section includes stationary X-shaped baffle plates that deflect the incoming pellet water slurry, such that the pellets intercept a screen member and are deflected, whereas the water passes through and is removed or recycled. The bulk dewatered pellets then pass downwardly out of the vertical dewatering section onto an angled feed chute comprising a screen member for additional dewatering and ultimately to pass the significantly dewatered pellet mass into the base portion of a dryer. Dewatering feed chute screens are also disclosed in U.S. Pat. Nos. 3,458,045; 4,476,019; and 4,896,435.
An agglomerate catcher and dewaterer is disclosed in U.S. Pat. No. 6,063,296, wherein the pellet slurry is introduced vertically into the agglomerate catcher including a downwardly angled grid of elongate bars or rods spaced a distance apart smaller than the agglomerate dimensions. The rods preferentially are disclosed with saw teeth along their base to further deflect water from the agglomerates. A secondary agglomerate catcher is connected to the principal dewaterer to further dewater the agglomerates prior to removal from the system. From the agglomerate catcher, the pellet slurry enters a cylindrical dewatering area where it is deflected by a cylindrical assembly to which is attached downwardly pointing conical deflectors periodically along its vertical height. The deflectors redirect the pellet slurry to a screen member from which the pellets are deflected and through which the water passes. Attached to the screen and below the downwardly pointing conical deflectors are attached inverted downwardly pointing conical annular rings. These redirect the pellets back toward the centrally located cylinder for additional impact dewatering and subsequent redirection toward the screen as above. The dewatered pellets pass through the base of the dewatering section into a dryer.
Centrifugal pellet dryers conventionally include a vertically disposed outer housing, a cylindrical screen oriented in the housing and a driven bladed rotor positioned centrally in the screen. The rotor moves water laden pellets or other particulates upwardly within the screen with upward and tangential velocity imparted to the particles by impact with the blades, causing the particles to move upward and tangentially outwardly into engagement with the screen for discharge from the upper end of the screen and housing, and water is discharged from the lower end of the housing.
Dryer equipment has been introduced and used in applications following extrusion and pelletization for many years by the assignee as demonstrated in, for example, U.S. Pat. Nos. 3,458,045; 4,218,323; 4,447,325; 4,565,015; 4,896,435; 5,265,347; 5,638,606; 6,138,375; 6,237,244; 6,739,457; 6,807,748; 7,024,794; 7,171,762; 7,421,802; U.S. patent application Publication Nos. 2006/0130353, 2008/0289208, 2009/0062427, 2009/0110833; World patent application Publication Nos. WO2006/069022, WO2008/113560, WO2008/147514, and WO2009/059020; German Patents and Applications including DE 19 53 741, DE 28 19 443, DE 43 30 078, DE 93 20 744, DE 197 08 988; and European Patents including EP 1 033 545, EP 1 602 888, EP 1 647 788, EP 1 650 516, EP 1 830 963. These patents and applications are all owned by the assignee and are included herein by way of reference in their entirety.
A cascade dryer using conical screen devices is disclosed in U.S. Pat. No. 3,199,215. The water pellet slurry enters the uppermost portion of the drier and bulk dewatering is accomplished in the uppermost chamber. The dewatered pellets pass through a conical screen into a sequence of conical screening devices such that the uppermost screen radiates downwardly from its apex and is of slightly smaller diameter than the subsequent and upwardly turned conical screen that is attached to an enclosure. The upwardly turned screen has a through opening in its center to the next sequence of conical screens. To facilitate drying, heated air is introduced near the base of the cascade dryer and moves upwardly through the multiplicity of conical devices.
U.S. Pat. No. 3,477,098 discloses a centrifuge type dryer wherein the pellet slurry is introduced into the center region of a rapidly rotating conical screen. U.S. Pat. No. 5,265,347 introduces the pellet slurry internal to the rotor but adjacent to the inner screen and lifter portion rather than into the central region. U.S. Pat. Nos. 5,611,150; 5,987,769; 6,505,416; and 6,938,357 disclose introduction of the pellet slurry through the center of the rotor, whereas U.S. Pat. Nos. 3,458,045; 4,476,019; 4,565,015; 4,896,435; 5,638,606; 6,438,866; and U.S. patent application Publication No. 2008/0072447 disclose the use of side feeding of the pellet slurry or the dewatered pellet mass into the base of the dryer external to the rotor. U.S. Pat. No. 4,476,019 further discloses a centrifugal dryer in which the rotor and screen assembly can be pivoted out of the housing for ease of access.
Tangential pellet outlets have been known to be highly effective in avoidance of build-up within the centrifugal dryers and as such have been disclosed exemplarily in U.S. Pat. Nos. 3,458,045 and 4,896,435.
Various rotor designs have also been disclosed including solid rotors wherein the cylindrical or tubular shell is essentially a single entity and segmented rotors wherein plates are attached to supports, the composite of which forms the rotor. Solid rotors are disclosed in U.S. Pat. No. 4,565,015 wherein it is described that a cylindrical hollow construction is supported by a web or strut elements. It is further disclosed that a rotor of welded construction or of square tubing has a reduced diameter, eliminates balancing issues, and is more rigid than rotors of a bolted construction. U.S. Pat. No. 5,987,769 similarly discloses an elongate tubular rotor pipe suspended within the interior of a screen member. U.S. Pat. Nos. 3,458,045; 4,218,323; 5,265,347; and 5,638,606 disclose use of various supportive structural elements or spiders to which are attached backplates essentially comprising the rotor.
In order for the rotor to effectively lift the pellets away from the remaining fluid up, through, and out of the dryer, various designs of blades have been disclosed. U.S. Pat. No. 4,565,015 discloses essentially rectangular-shaped angled lifting blades vertically along the length of the rotor as well as radial blades on the uppermost portion of the rotor designed to redirect the pellets from the rotor out the pellet outlet chute and away from the drying apparatus. U.S. Pat. No. 5,987,769 discloses the use of blades that are illustrated as essentially rectangular shaped affixed to the rotor in an angular configuration. Between these blades are linear blades parallel to and along the length of the rotor axis—described as kickers. Scraper blades, disclosed as being L-shaped are attached to the uppermost portion of the rotor to deflect pellets out and away from the dryer. The scraper blades are disclosed in alignment with the angular blades as well as the linear or kicker blades. U.S. Pat. No. 6,438,866 similarly discloses a linear deflector blade in combination with the lifter blades with the linear deflector blade illustrated angularly oriented back from the attachment points as viewed in the direction of rotation of the rotor.
The lifter blades disclosed in U.S. Pat. No. 3,675,697 include two components one of which is essentially planar and perpendicular to the longitudinal or vertical axis of the rotor, and a second essentially triangular component attached to the planar component and pointed upwardly and angularly toward the planar component of blades in the next higher row. U.S. Pat. No. 6,505,416 essentially identifies four regions of lifter blades along the vertical height of the rotor. The initial or lowermost section of blades essentially form an auger style portion allowing sufficient open area for the pellet slurry introduced between these blades as it is delivered from the center of the rotor. The second more heavily populated portion of the rotor is in the dewatering section to provide additional impacts to insure that the pellets can be removed from the incoming fluid of the slurry and transported up and through the drying, or third, portion where the number of blades is significantly reduced. The uppermost or fourth portion has blades oriented parallel to the vertical axis of the rotor to deflect the significantly dried pellets out of and away from the dryer. The blades other than those in the fourth section are preferably involute in structure allowing for curvature of the outer blade edge upwardly and towards the rotor to deflect the pellets toward the underside of the next row of blades and reduce the impacts directly onto the screen as these are deemed detrimental to the quality of the pellets.
The housings of conventional dryers have been round, square, and hexagonal as disclosed in U.S. Pat. Nos. 5,987,769; 4,476,019; and 5,265,347 respectively. Similarly, many and various types of screens have been utilized, from hinged screens as disclosed in U.S. Pat. No. 5,265,347, to multilayer screens as disclosed in U.S. patent application Publication No. 20060130353. Deflectors or flow disrupters have also been disclosed for use on the screens such that banding of pellets about the screen can be avoided. These disclosures include U.S. Pat. No. 6,438,866 wherein angled attachments as well as angled blocks are incorporated at the juncture of the screen components. Deflector bars are attached directly to non-screen portions of the screen components as disclosed in U.S. Pat. No. 6,739,457. U.S. patent application Publication No. 2008/0289208 further discloses that the deflectors can be embossed into non-screen portions of the screen components. Various portions of the dryer can be treated with abrasion-resistant non-stick surface treatments as disclosed in World Patent Application Publication No. WO2009/059020.
Surprisingly, with all these variants and attempts at dryer systems, a reliable consistent throughput dryer remains elusive, particularly one that has high throughput and low moisture content without compromising facile cleaning, manually or automatically, of an agglomerate catcher and without over-powering the dewatering components of the dryer. Furthermore, little mention of the possible impact of lifter positioning and orientation has been discussed.